1. Field of the Invention
The present application relates to a DC-DC converter.
2. Description of Related Art
A DC-DC converter is controlled by switching operational methods between a PFM (Pulse Frequency Modulation) method and a PWM (Pulse Width Modulation) method. The typical DC-DC converter is operated using the PWM method in normal operational states, including at heavy load times, and is switched to the PFM method at light load times.
FIG. 1 shows a DC-DC converter 100 of the related art. In FIG. 1, an error amplifier ERA101 of a control circuit 111 amplifies the difference in voltage between 1) an output voltage Vout of the DC-DC converter, as divided by a resistive element R101 and a resistive element R102, and 2) a reference voltage e101, to produce an output voltage Vc. In operation, for example, if the output voltage Vout rises, the difference between Vout and the reference voltage e101 becomes smaller, and the output voltage Vc falls. Further, if the output voltage Vc becomes lower than a reference voltage e102, a voltage comparator COMP102 outputs a low-level output voltage Vd2.
As shown in FIG. 1, an AND gate circuit AND102 get through a pulse signal PS when a high-level output voltage Vd2 is input from the voltage comparator COMP102, and inputs an output into a set terminal S of a flip-flop FF as a set signal SS. In addition, the AND gate circuit AND102 masks pulse signal PS when the low-level output voltage Vd2 is input. As a result, switching operation of the DC-DC converter 100 is skipped. This function can avoid a rise in output voltage by preventing a transistor FET101 from turning on when the load becomes lighter and the output voltage Vout of the DC-DC converter 100 rises above a specified value. Meanwhile, because the transistor FET101 switches at a different frequency from the oscillator frequency of an oscillation OSC, operation of the PFM may be controlled.
FIG. 2 shows a timing chart for the DC-DC converter 100 of the related art of FIG. 1. As shown in FIG. 2, the DC-DC converter 100 masks the pulse signal PS, depending on the output voltage Vout. In operation, this is accomplished by the DC-DC converter directly controlling ON/OFF switching, depending on the output voltage Vout. As FIG. 2 shows, switching operations continuously occur until the output voltage Vout reaches a specified voltage, at which time the switching operation ceases until the output voltage Vout becomes lower than a reference voltage. As a result, the switching operation might “localize” as shown in area R100. The occurrence of this localization phenomenon can be a problem in the related art because a noise can be generated, or a ripple in the output voltage Vout can occur or increase. Localization of this switching phenomenon may also be a problem because it can thereby be generated even in static states, appearing as noise, where load is generally constant.
There remains a need in the related art for control a circuit for DC-DC converters that is useful to avoid localization of switching operation at light loads and that can improve power conversion efficiency.